Nitrogen deficiency diminishes consumption of photosynthates in anabolic metabolism. We studied adjustments of the photosynthetic machinery in nitrogen-deficient bean plants and found four phenomena. First, the number of chloroplasts per cell decreased. Chloroplasts of nitrogen starved leaves contained less pigments than those of control leaves, but the in vitro activities of light reactions did not change when measured on chlorophyll basis. Second, nitrogen deficiency induced cyclic electron transfer. The amounts of Rubisco and ferredoxin-NADP(+) reductase decreased in nitrogen starved plants. Low activities of these enzymes are expected to lead to increase in reduction of oxygen by photosystem I. However, diaminobenzidine staining did not reveal hydrogen peroxide production in nitrogen starved plants. Measurements of far-red-light-induced redox changes of the primary donor of photosystem I suggested that instead of producing oxygen radicals, nitrogen starved plants develop a high activity of cyclic electron transport that competes with oxygen for electrons. Nitrogen starvation led to decrease in photochemical quenching and increase in non-photochemical quenching, indicating that cyclic electron transport reduces the plastoquinone pool and acidifies the lumen. A third effect is redistribution of excitation energy between the photosystems in favor of photosystem I. Thus, thylakoids of nitrogen starved plants appeared to be locked in state 2, which further protects photosystem II by decreasing its absorption cross-section. As a fourth response, the proportion of non-Q(B)-reducing photosystem II reaction centers increased and the redox potential of the Q(B)/Q(B)(-) pair decreased by 25 mV in a fraction of photosystem II centers of nitrogen starved plants.